Some sensors and other components of spacecraft and aircraft must be cooled to cryogenic temperatures of about 77.degree. K or less to function properly. A number of approaches are available, including thermal contact to liquefied gases and cryogenic refrigerators, usually termed cryocoolers. The use of a liquefied gas is ordinarily limited to short-term missions. Cryocoolers typically function by the expansion of a gas, which absorbs heat from the surroundings. Intermediate temperatures in the cooled component may be reached using a single-stage expansion. To reach colder temperatures required for the operation of some sensors, such as about 40.degree. K or less, a multiple-stage expansion cooler may be used. The present inventors are concerned with applications requiring continuous cooling to such very low temperatures over extended periods of time.
One of the problems encountered in some applications is that the total heat load which must be removed by the cryocooler, from the object being cooled and due to heat leakage, may vary over wide ranges during normal and abnormal operating conditions. The heat loading is normally at a steady-state level, but it occasionally peaks to higher levels before falling back to the steady-state level. The cryocooler must be capable of maintaining the component being cooled at its required operating temperature, regardless of this variation in heat loading and the temporary high levels. While it handles this variation in heat loading, the cryocooler desirably would draw a roughly constant power level, so that there are not wide swings in the power requirements that would necessitate designing the power source to accommodate the variation.
One possible solution to the problem is to size the cryocooler to handle the maximum possible heat loading. This solution has the drawback that the cryocooler is built larger than necessary for steady-state conditions, adding unnecessarily to the size and weight of the vehicle. Such an oversize cryocooler also would require a power level that varies widely responsive to the variations in heat loading.
There is a need for an improved approach to the cooling of sensors and other components to very low temperatures. The present invention fulfills this need, and further provides related advantages.